Samsung S5PC100 User Manual page 315

BUS CONFIGURATION
2.3.1 Least Recently Granted (LRG) Scheme
In the LRG scheme, each connected SI has a single slot associated with it, but each interface also has a priority
value. This priority value, whose post-reset value can be configured at design time and programmed or
interrogated through the APB programming interface, can make the arbiter behave as:
•
A pure LRG scheme
•
A fixed priority encoder
•
A combination of the two.
All masters with the same priority form a priority group. As a result of arbitration, a master can move within its
priority group but cannot leave its group, and no new masters can join the group.
Arbitration is granted to the highest priority group from which a member is trying to win access, and within that
group, to the highest master at that time. When master wins arbitration, it is relegated to the bottom of its group to
ensure that is cannot prevent other masters in its group from accessing the slave.
If you configure all master priorities to different levels, the arbiter implements a fixed priority scheme. This occurs
because in this case, each master is in a group of its own, and therefore, masters maintain their ordering.
If all master priorities are the same, then an LRG scheme is implemented. The reason that it behaves as an LRG
is because the process of relegating the master that was last granted access, to the bottom of its group, results in
the masters being ordered from the LRG master at the top, to the Most Recently Granted (MRG) at the bottom.
The LRG and fixed priority modes concurrently exist when the master priority value registers are programmed
with a combination of identical and unique values. You can mix priority groups that contain one member with
priority groups that contain more than one member in an arbitrary manner. The arbiter places no restriction on the
number of groups or their membership.
Figure 3.1-4 shows the movement of masters within their priority groups.
3.1-6
Figure 3.1-4 Example Operation of LRG Arbitration Scheme
S5PC100 USER'S MANUAL (REV1.0)